Method of treating spherical powder grains

ABSTRACT

A method of treating powder granules to remove a skin present on their outer surface due to a protective colloid of animal origin, said method comprising suspending the powder granules in a mixture of an aqueous solution and a proteolytic enzymes, and agitating the suspension until the skin is removed.

United States Patent 1 Coffee Jan. 30, 1973';

[54] METHOD OF TREATING SPHERICAL POWDER GRAINS Ralph E. Coffee, Alton, ll].

Olin Mathieson Chemical Corporation Filed: Sept. 25, 1968 Appl. No.: 762,641

Inventor:

Assignee:

(1.8. CI ..264/3 D, 149/10, 149/96, 264/3 R, 195/2 Int. Cl. ..C06b 21/02 Field of Search ..l95/2, 3, 4; 264/3; 149/2, 149/9, 10, 96, 97, 98; 260/223 [56] References Cited I UNITED STATES PATENTS 2,027,l l4 H1936 Olsen et al ..l49/2 3,400,026v 9/1968 Fearnow 149/2 Primary ExaminerA. Louis Monacell Assistant Examiner-Gary M. Nath Attorney-H. Samuel Kieser, Donald R. Motsko, William W. Jones and Richard S. Strickler 57 ABSTRACT 9 Claims, No Drawings METHOD OF TREATING SPHERICAL POWDER GRAINS This invention relates generally to the manufacture of propellant powder and more particularly to a method of treating spherical or near spherical grains of smokeless powder.

In U.S. Pat. No. 2,027,114, granted Jan. 7, 1936 there is disclosed a process of manufacturing smokeless powder wherein droplets of lacquer, comprising a smokeless powder base and solvent, are solidified while suspended in a non-solvent medium and a protective colloid. Such a process for manufacturing propellant powder has come to be known to those skilled in the art as the globular powder process.

Propellant powder grains made by the globular powder process have been used extensively for casting solid rocket propellants. The use of propellants in the form of a rod, a rod with internal star, hollow rod, multiple port rod, and the like is well known in the art for the purpose of controlling thrust throughout the combustion period.

It has been observed that in many'instances solid propellants cast from powder grains produced by the globular powder process have relatively low propellantinhibitor bond strength (peel strength) and low tensile strength as compared to propellants cast from powder grains made by a different process, such as extruded grains. In certain cases, wherein the propellants were cast from powder grains produced from the globular powder process, these has been a failure of the propellant-inhibitor bond upon firing of the rocket which results in uncontrolled burning of the propellant during the combustion period. Such uncontrolled burning not only results in overpressurization causing undesirable variations in the thrust program, but also may cause rupture or explosion of the rocket vessel.

it has been observed that powder grains produced by the globular powder process have a skin present on their outer surface. It is theorized that this skin is caused by the retention by strong adsorption at the surface of protective colloids of animal origin which are used in the process. This skin adversely affects the propellant-inhibitor bond strength, as well as the tensile strength, of propellants cast from such powder grains.

It is an object of this invention to provide a method of treating casting powder produced by the globular powder process to overcome the above-mentioned difficulties.

A more specific object of this invention is to provide a method for treating globular powder which results in an increase in the propellant-inhibitor bond strength and tensile strength of a propellant cast from such powder.

Yet another object of this invention is to provide a method for removing the skin from globular powder grains caused by the adsorption of animal origin colloids.

The invention contemplates treating globular powder grains with an enzyme which will hydrolyze either protein or fats. Since this skin is of animal origin and may contain a wide variety of fats and proteins, this invention further contemplates a mixture of enzymes including proteolytic enzymes which are substances which have the ability to attack proteins and effect hydrolysis to amino acids and lipases which are enzymes that effect the hydrolysis of fat.

More in detail, the powder grains to be treated are prepared in accordance with the conventional globular powder process. Generally, in such process, a lacquer is prepared consisting of a suitable powder base, such as nitrocellulose, dissolved in a solvent. Various solvents suitable for use with nitrocellulose include methyl ethyl ketone, ethyl formate, isopropyl acetate, diethyl ketone, ethyl acetate and mixtures thereof.

The lacquer prepared in accordance with the above.- identified procedure may be presized if desired and then admixed with a non-solvent suspending material. Although the suspending material may be any liquid that is substantially immiscible with the lacquer, an aqueous medium is generally preferred. The non-solvent material also contains a protective colloid and/or other additives which are described more fully below.

The protective colloids are preferably of animal origin such as the type derived from animal protein, animal bone glue, or the like. Chemically each of these protective colloids is a collagen which by definition is an albuminoid comprising the major portion of the white fiber in the connectivetissues of the animal body, particularly in the skin, bones and tendons. This colloid may be present in the non-solvent material in a proportion equivalent to between about 0.1 and about 0.5 percent by weight.

Liquid high energy compounds such as triethylene glycol dinitrate, diethylene glycol dinitrate, and nitroglycerin may be added with the lacquer to the suspending material in a proportion up to about 50 percent by weight of the powder base in order to increase the burning rate of the solid powder grains ultimately produced and to serve as a plasticizer when the powder is used as a component of solid propellants. In addition a crystalline high explosive composition in finely divided form may be added to the lacquer or solvent material prior to or during mixing. Any crystalline high explosive composition which is substantially insoluble in the water immisciblesolvent used in preparing the lacquer of nitrocellulose can be used in thepreparation of the novel rocket cast granules of this invention. Typical examples of suitable explosive compositions include HMX (cyclotetramethylenetetranitramine), RDX (cyclotrimethylenetrinitramine), PETN (pentaerythritol tetranitrate), 2,4,6-trinitrophenylmethylnitramine, and mixtures thereof.

Any suitable mixing vessel provided with a mechanical agitator or other mixing device may beemployed. The mixture is kept under agitationsufficient to maintain the state of suspension until the suspended presized bodies of lacquer become rounded under the force of interfacial tension. Y

Mixing of the lacquer and suspending material, and the formation of the globules may be carried out ata temperature between about 10C and about C, or any temperature above the freezing point of the mixture and below the boiling point of the volatile solvent. Pressures above atmospheric pressure may be employed during mixing to inhibit evaporation of the solvent at higher temperatures.

Agitation of the lacquer in the suspending material for a period of at least about one-half hour, and as long as 1 hours or more, is generally necessary to form globules of the desired size and shape and, if utilized, to effect substantially complete dispersion of the particles of highly explosive composition or other solid additives in the lacquer globules.

If desired, as the rounded globules of lacquer are formed and become substantially uniformly dispersed in the nonsolvent material, an additional amount of solvent for the nitrocellulose may be slowly added for agitation with the suspension. This addition of solvent causes a breakdown in the lacquer globules and additional lacquer globules are formed having a diameter substantially less than the original globules.

The solvent may then be evaporated from the lacquer globules by increasing the temperature of the suspension up to the distillation temperature by bubbling air or other inert gas through the suspension. If the specific gravity and porosity of the individual powder grains are not critical, the evaporation may be continued until complete solidification of the lacquer globules is obtained whereupon the resulting solids are then separated from the non-solvent material for further processing in accordance with this invention. If the specific gravity and porosity of the powder grains is critical, additional processing may be performed prior to the final evaporation and complete solidification of the globules.

The above description of the globular powder process is intended to be merely a brief outline of such process and it should be understood that the present invention is applicable to any powder grains produced by any of the various modifications of the globular powder process in which a protective colloid of animal origin is utilized.

in accordance with this invention, powder grains produced by the globular powder process are further treated with an enzyme or mixture of enzymes which will hydrolize proteins and/or fats which are in the skin of the powder particles due to the protective colloid. Generally, any proteolytic enzymes, any of the lipases, or combinations of the two types may be used. Among the enzymes which are particularly effective are pepsin, papain, those enzymes produced by aerobic bacteria, trypsin, and pancreatin. Also, Serizyme, which is a trademark for a standardized bacterial proteolytic enzyme preparation of the Wallerstein Co. may be used.

The powder grains to be treated may be loaded into a vessel equipped with an agitator along with suitable quantity of water. The agitation is started and run at a sufficient speed to keep the powder grains in suspension in the water.

The proportion of water in relation to the powder grains may vary widely with the only requirement being that there must be enough water present to suspend the powder grains. In the preferred embodiment, one part by weight powder grains to three parts by weight water is used.

After the powder grains are suspended, a suitable amount of enzyme is added to the vessel. it has been found that the proportion of the enzyme to the powder grains may also vary widely. However, it must be noted that the less enzyme solution used, the greater the length of time the treatment must be continued.

It has been found that nine parts by weight of a suitable enzyme to 100 parts by weight powder grains is effective when the treatment is continued for 4 hours at 40C. Although 40C is the preferred temperature, the temperature at which the mixture is agitated may range from ambient to 50C. At temperatures above 50C, the enzyme loses its potency and-temperatures below ambient are not desirable because of the excessive time required for treatment.

As a specific example, l00 parts by weight of powder grains produced by the globular powder process were loaded into a vessel equipped with an agitator along with 300 parts by weight of water. During the process of making the globular powder, Swift Colloid No. l was used as the protective colloid. The agitation was started and continued at a sufficient speed to keep the powder grains in suspension. The enzyme Serizyme, in the proportion of nine parts by weight enzyme to parts by weight powder grains, was added to the vessel and the temperature raised to 40C. The agitation was continued for 4 hours, after which the powder was separated from the treatment material in a centrifuge, washed with clear water to remove the residual material, and dried.

Test specimens were prepared by mixing the treated powder grains with a solution of 60 parts by weight nitroglycerin and 40 parts by weight dimethyl sebacate in a ratio of approximately 69 parts by weight of powder to 31 parts by weight of the solution. The resulting slurry was cast into a cylindrical mold, cured at F for 48 hours, and then machined to produce a dumbbell-shaped specimen. The average of standard tensile tests performed on the specimens was a tensile strength of 620 p.s.i.

As a comparison, powder grains from the same run of the globular powder process, but not treated with the enzyme, were mixed with a solution of nitroglycerin and dimethyl sebacate, cast, cured, and machined to provide a dumbbell-shaped specimen in the same manner as described above. The average results of the tensile tests performed on these specimens were a tensile strength of 410 p.s.i., significantly less than that exhibited by specimens prepared with powder grains treated with the enzyme.

To test the peel strength, powder grains produced by the same globular powder process and treated with the enzyme Serizyme in the manner described above were mixed with a solution of 60 parts by weight nitroglycerin and 40 parts by weight dimethyl sebacate. The mixture was approximately 69 parts by weight powder to 31 parts by weight solution. The mixture was cast into a mold having a cellulose acetate inhibitor and cured to bond the propellant thereto. After curing, the propellant with the cellulose acetate inhibitor was mounted in a testing machine and the propellant peeled from the inhibitor. The average of the tests performed was 35.7 pounds per inch.

in a similar manner, specimens prepared from powder grains produced by the globular powder process, but untreated with an enzyme, were tested to provide a comparison. The results of the tests performed upon such specimens were 17 pounds per inch, less than half that exhibited by the specimens prepared with grains treated in accordance with this invention.

It is to be understood that the foregoing description is merely illustrative of the many modifications and variations that will present themselves to those skilled in the art. Accordingly, it is intended that the scope of this invention be ascertained from the following claims.

I claim:

1. A method of treating powder granules that have a skin present on their outer surface due to a protective colloid of animal origin being present during the formation thereof, said method comprising suspending said powder granules in a mixture of an aqueous solution and a proteolytic enzyme, agitating said suspension until substantially all of said skin is removed, and

thereafter separating said powder granules from the remainder of said materials.

2. The method of claim 1 wherein said agitation is carried out at a temperature between ambient and 50C.

3. The method of claim 1 wherein the suspension comprises one part by weight powder grains to 3 parts by weight water and 9 parts by weight enzyme to 100 parts by weight powder grains.

4. The method of claim 3 wherein said suspension is agitated at a temperature of from ambient to 50C.

5. The method of claim 4 wherein said suspension is agitated at a temperature of 40C for 4 hours..

6. A method of treating globular powder granules that have been produced by suspending droplets of lacquer of a smokeless powder base and a solvent in a non-solvent medium and a protective colloid of animal origin and then solidified into granules of a globular shape, having a skin on their outer surface, said method comprising suspending said solidified globular powder granules in a mixture of an aqueous solution and a proteolytic enzyme, agitating said suspension until substantially all of said skin is removed, and thereafter separating said powder granules from the remainder of the materials.

7. The method of claim 6 wherein saidagitation is carried out at a temperature between ambient and 50C.

8. The method of claim 6 wherein the suspension comprises one part by weight powder grains to 3 parts by weight water and 9 parts by weight enzyme to I parts by weight powder grains.

9. The method of claim 8 wherein said suspension is agitated at a temperature of from ambient to 50C.

II l 

1. A method of treating powder granules that have a skin present on their outer surface due to a protective colloid of animal origin being present during the formation thereof, said method comprising suspending said powder granules in a mixture of an aqueous solution and a proteolytic enzyme, agitating said suspension until substantially all of said skin is removed, and thereafter separating said powder granules from the remainder of said materials.
 2. The method of claim 1 wherein said agitation is carried out at a temperature between ambient and 50*C.
 3. The method of claim 1 wherein the suspension comprises one part by weight powder grains to 3 parts by weight water and 9 parts by weight enzyme to 100 parts by weight powder grains.
 4. The method of claim 3 wherein said suspension is agitated at a temperature of from ambient to 50*C.
 5. The method of claim 4 wherein said suspension is agitated at a temperature of 40*C for 4 hours.
 6. A method of treating globular powder granules that have been produced by suspending droplets of lacquer of a smokeless powder base and a solvent in a non-solvent medium and a protective colloid of animal origin and then solidified into granules of a globular shape, having a skin on their outer surface, said method comprising suspending said solidified globular powder granules in a mixture of an aqueous solution and a proteolytic enzyme, agitating said suspension until substantially all of said skin is removed, and thereafter separating said powder granules from the remainder of the materials.
 7. The method of claim 6 wherein said agitation is carried out at a temperature between ambient and 50*C.
 8. The method of claim 6 wherein the suspension comprises one part by weight powder grains to 3 parts by weight water and 9 parts by weight enzyme to 100 parts by weight powder grains. 